Bob kb8tq wrote: >Hi > >Without the ability to put out a known good time pulse there is no quick way >to >check NTP. GPS modules suffer a similar issue. They put out a pulse and a >correction (sawtooth error) to tell you what they just told you. Doing the >same >sort of thing with NTP may be possible. > >Indeed the process of correcting this sort of data is open to a bit of debate. >It does >give you a way to get around the hey, all I can do is 300 ns issue. With >GPSDOs >the correction is part of the standard firmware. It would be nice if one of >the NTP >gurus popped up with an equivalent process. > >One *could* monitor various bits and pieces of the OSs timing generation >system. >Somehow that does not seem quite as much fun as looking at the whole result all >at once. Indeed it might be the only way to get it all worked out. > >Bob
Linux has a pps output driver (pps_gen_parport), but I've never used it. A while back I added an output mode to the Linux pps_parport driver: https://github.com/retroman/pps_parport_polled that I will eventually get around to using with the palisade(trimble) NTP driver. My modified driver's polled input mode has an input-to-echo delay of 1.16 to 1.93 microseconds (measured with an old Keithley 775 counter) on my machine which has a parallel card attached directly to the pci-e port on a sandybridge processor. Interrupt mode echo delay is 3.8 to 4.3 microseconds when the machine is lightly loaded with occasional spikes of 5 to 7 us. When the machine is idle delay falls to 3.5-4.1us. Port read and write delays are equal at about 820ns each. I think that pci-express always uses 'split transactions' so reads can sometimes seem to take only half the time depending on the input pulse time relative to the start time of a read request. Delays increase to ~1200ns when attached to the chipset pcie ports. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
